I imagine the main reason is that catering to disabilities is much much more expensive when you consider the variety of disabilities that there are and how much they differ in severity and how they will affect different individuals in different ways.
I also guess that people with more severe disabilities are less likely to control large corporate or personal budgets than the general population and those that do are either very good at working around their disability of simply have others to who do it for them.
It's also possible that making a site easier to use for somebody with one disability might have the effect of making it worse for a person with a different disability.
I'm sure it is quite expensive but then again, as we a web developer, my job is easily made 50% more difficult (thus VERY expensive) when I have to support IE.
The problem as I see it--and to your point--is that I work on enterprise web apps and my Fortune 500 customers have never said a word about accessibility for some reason. I'm sure they honor their Americans with Disabilities Act obligations and build ramps into their buildings. Do they then not care about employees (or customers and vendors for that matter) once they're at their desks?
I don't even think it's possible to make a website that caters to all disabilities. What is awful is that a lot of the most common disabilities (visual and hearing impairments, etc.), that affect a lot more people than you might think, could be accommodated pretty damn easily and cheaply if anyone was aware and cared in the least. But they aren't aware and they don't care, or they laugh it off like being a decent human being and giving a shit is a joke!
I also guess that people with more severe disabilities are less likely to control large corporate or personal budgets than the general population and those that do are either very good at working around their disability of simply have others to who do it for them.
It's also possible that making a site easier to use for somebody with one disability might have the effect of making it worse for a person with a different disability.